The answer to keeping the bedding, hay, and baby guinea pigs in the cage. Especially since my grids have 1.5" openings. Mosquito netting from the camping section of a sports store.

It's actually less noticeable when the flash isn't shining on it. At first glance you don't even notice it on the outside of the cage.
Here's baby Peppermint to model the roll of netting.

Well she actually thought wandering off to explore was more interesting than looking at the camera.
Rolo already tasted it while I was cutting it to size and said it wasn't worth chewing on. Now that it's on the cage they haven't shown any interest in reaching through the bars. If they did this stuff wouldn't last long because it's very fine. I more just slid the scissors along it instead of actually cutting it but I can't pull it apart with my hands so it should hold up fine.